1. Field of the Invention
The present invention relates to a dry toner used to develop electrostatic latent images in electrophotographic and electrostatic recording processes, a process for the preparation thereof, a developer comprising the said toner, and a method of forming images.
2. Description of the Related Art
In the electrophotographic process, an electrostatic latent image formed on a photoreceptor comprising a photoconductive material is developed by applying a toner onto the electrostatic latent image by means of, e.g., a magnetic brush. The toner on the photoreceptor is transferred onto a transfer medium such as a paper or sheet, and the transferred toner is fixed by means of heat, solvent, pressure or the like to produce an image. To repeatedly provide copied images of high quality, therefore, it is essential for each step to fully operate.
For instance, it is important to generate a uniform electric field near the transfer medium in the transfer step. Conventionally, Corotron method is widely used to generate the electric field due to its simple mechanism and low cost.
However, the Corotron method has various problems: upon electrical discharging, ozone, harmful to humans, is produced; a high voltage power supply is required in the copying process and thus regular maintenance is necessary for cleaning discharge products. Accordingly, a bias roll transfer method capable of solving the above disadvantages has been proposed and studied. In this transfer method, a transfer medium is interposed between a photoreceptor and a bias roll, and a transferring electric field is applied directly to the transfer medium by the bias roll. To transfer a toner, it is necessary that the transfer medium and bias roll are in contact with each other at a linear load of 5 g/cm or higher. When pressure is applied between the transfer medium and bias roll, however, the toner image on the photoreceptor is also pressurized and, consequently, toner particles in the toner image may aggregate and the toner image may adhere or stick to the photoreceptor. As a result, a part of the toner image on the photoreceptor is not transferred onto the transfer medium and"incomplete image", i.e., incomplete image transfer onto the transfer medium, occurs more often.
To prevent such problems, a method for adding finely divided powder treated with a silicone oil or varnish to the surface of toner particles has been proposed (Japanese Patent Application Laid-Open (JP-A) No. 3-121462) . According to this method, the incomplete image problem can be prevented initially but not fully prevented upon prolonged use. In particular, the incomplete image tends to readily occur when plain paper is used as a transfer medium under high temperature, high humidity environments and when an OHP sheet is used as a transfer medium under low temperature, low humidity environments. Such defects are suppressed by an external additive and the toner itself is not fundamentally improved in this method. Thus, it is necessary to prevent the incomplete image by improvement of the actual toner.
In addition to the defect of image quality in the transfer step, various problems in the fixing step following the transfer step must be solved in order to provide copied images of high quality.
The heat melt method is most often used in fixing toner images. In particular, the contact heat roll fixing method has a good thermal efficiency enabling high speed fixing and, therefore, at present it is more widely used in commercial copying machines and printers than the pressure roll fixing method. However, the heat roll fixing method has the following disadvantages or problems.
1) Recently, there has been a strong need for saving electric source in copying machines and printers from the viewpoint of resource saving. Of all the various steps in the whole apparatus, the fixing step consumes most electric source. In particular, the energy, i.e., the electric source required in the heat roll fixing method is significantly larger than the energy expended in the pressure roll fixing method. For saving energy, therefore, it is important to reduce the minimum temperature required to fix the toner (reduction of fixing temperature)
2) Offset phenomena wherein the toner adheres to the heat roll and soils the subsequent copy readily occurs, in particular when the fixing speed is high, and its prevention is desirable (prevention of offset phenomena).
3) Near the fixing roll there is provided a scraper for preventing the transfer medium (usually paper) from winding around the roll after having passed therethrough. If a copying machine is run at a high speed, the stress exerted on the scraper increases and consequently poor scraping or image deficiency at tip portions of the transfer medium by the scraper may occur. It is expected that such disadvantages can be fully obviated even when the fixing temperature is low (improvement of releasability at low temperatures and prevention of scratches by a scraper).
4) When copying the double-sided original, multi-color original or copied images, a toner image of the resulting copy is rubbed by a paper feed roller in an automatic original feeder or copying machine or by the reverse side of an upper copy when superimposed to produce stain or blur resulting in reduction of image quality. A toner which forms images resistant to rubbing is expected (improvement of rubbing resistance).
5) Prevention of reduction of flowability of the toner which causes aggregation (blocking) thereof resulting in lowering storage stability and carrier of toner is another problem (improvement of flowability and prevention of aggregation).
To solve some of these problems, the following attempts to improve toners by improving their components, lubricants and binder resins have been carried out.
For example, addition of low molecular weight polypropylene or polyethylene as a lubricating component of the toner has been proposed: Japanese Patent Application Publication (JP-B) Nos. 52-3304, 52-3305, 57-52574 and 58-58664; and Japanese Patent Application Laid-Open (JP-A) Nos. 58-59455 and 60-151650.
The use of these lubricating components improves to some extent offset resistance, prevention of scratches by the scraper, and rubbing resistance of fixed images but does not improve them satisfactorily. When a low molecular weight polypropylene is added as a lubricant of the toner, the temperature in the fixing step can not be lowered due to the high melting point of the polypropylene. In the case of toners to which a low molecular weight polyethylene is added as a lubricant, on the other hand, the flowability and aggregation resistance of the toners are poor, particularly when the amount of polyethylene added to the toners is increased to improve the offset resistance.
The use of a binder resin having Tg (glass transition temperature) a few dozens lower than Tg of conventionally used binder resins or a low molecular binder resin is effective in reducing the minimum temperature required to fix a toner. However, the range of fixing temperatures will simultaneously be lowered as a whole, resulting in reduction of hot offset resistance. In particular, the increase of the amount of heat per unit time to enable high speed copying causes more readily the offset phenomenon than usual copying. Further, the releasability at low temperatures such as prevention of scratches by a scraper can not be improved.
The use of a wax for improving the releasability at low temperatures together with a binder resin permitting minimum fixing temperature reduction has been proposed. Although polypropylene wax is often used as the wax in view of the balance between the improvability and other properties, its melting point is relatively high at about 145.degree. C., and may cancel the effect of lowering the minimum fixing temperature which effect is provided by the selected binder resin.
Therefore, a lower melting point wax is added together with the binder resin: Japanese Patent Application Laid-Open (JP-A) Nos. 3-17661, 4-97163 and 7-287413. This method is roughly divided in two cases: a wax having a lower melting point in its usual state such as natural wax or the like is added; a polyolefin wax having an artificially reduced melting point is added. In both cases, the effect of lowering the minimum fixing temperature provided by the improvement of a binder resin and/or lubricant is not canceled and the wax can melt at a lower temperature than the minimum fixing temperature of the binder resin. Thus, the releasability in the fixing temperature range can be improved to some extent. However, the wax may readily cause significant reduction of the flowability and aggregation resistance of the toner due to the low molecular weight component contained therein. In the case of the natural or similar wax, the rubbing resistance of copied images after fixing can not be improved to the extent that resistance is improved with the polyolefin wax due to the uneven molecular structure of the former.
To obviate these inconveniences (reduction of flowability or aggregation resistance), the use of a wax having both a low melting point and an average molecular weight within a specific range and a specific molecular weight distribution (free of any low molecular component) has been proposed: Japanese Patent Application Laid-Open (JP-A) Nos. 6-67455 and 6-75422. These method can reduce to some extent the adverse effect of the low melting point toners on the flowability and aggregation resistance. However, some such waxes may rather reduce the releasability at low temperatures unless some other properties of the waxes are sacrificed.
In short, there is a demand for a method of obviating the above problems 1) to 5) in an effective, overall and well balanced way.